A. Field of the Invention
This invention relates to velocity compensation for limited-displacement motors such as those used in galvanometers and like instruments. More specifically, it relates to a velocity-sensing circuit used to provide electronic damping for a limited-displacement motor.
The motors with which the invention is principally concerned are rotational devices used in instrument applications such as galvanometers and pen motors for strip chart recorders. Instruments of this type undergo a limited angular displacement in response to an electrical input. To make the displacement closely correspond with the input signal, the motors are often connected in closed-loop circuits incorporating position feedback. For circuit stability, these circuits often include electronic damping means of velocity feedback. Damping is also desirable in open loop operations to increase the upper frequency limit of the motor and this again requires a signal indicative of the velocity of the motor.
Conventionally, a velocity signal is provided by an electromechanical velocity transducer mechanically coupled to the motor or incorporated therein. A transducer of this type adds materially to the cost of the instrument. In closed-loop systems the velocity signal can be derived by differentiating the output of the position transducer that provides the position feedback signal. However, this often presents noise problems which are handled with difficulty.
A different type of velocity signal is described by M. Kanner, in Electronic Design, Dec. 19, 1968, pp. 100-101. The generator, which is used in a speed-control circuit for a continuous-rotation motor, makes use of the fact that the motor itself develops a voltage proportional to velocity. The motor is incorporated in a bridge circuit with three resistors whose values are arranged to balance out the resistance of the motor armature. The unbalance voltage developed by the bridge is then due solely to the velocity-dependent signal generated by the motor. The present invention is directed to a circuit in which a limited-rotation-motor generates the velocity-indicating signal.